1. Field of the Invention
Embodiments of the present invention relate to adhesive compositions containing thermally-labile groups. In particular, thermally degradable, epoxy-based adhesive compositions suitable for rework use in electronic component surface mounting applications are provided.
2. Description of the Related Art
Epoxy-based adhesives are thermosetting materials routinely employed in the electronics industry as encapsulants or underfill for electronic components, e.g., integrated circuit (IC) devices, surface mounted onto printed circuit (PC) boards. Epoxy-based adhesives may be used as an encapsulant to provide a rigid, hermetic seal for electronic components to protect the input/output metallic leads of the device from corrosion or other environmental conditions. Epoxy-based adhesives are also used as an underfill to maintain the mechanical integrity of the solder joints attaching the leads of the device to the bond pads of the PC board. Due to the difference in the coefficients of thermal expansion of the materials used in fabricating the device (e.g., ceramic IC packaging with metallic leads) and the PC board (e.g., polymeric), the soldered leads experience mechanical stress as a result of temperature changes or thermal cycling over the life of the device. Over time, these mechanical stresses may lead to device failure as a result of breakage of one or more soldered leads. To prevent a shortened fatigue life of a surface-mounted device, an underfill is applied to reinforce the soldered joints between the device and the PC board. The composition of the epoxy-based adhesive underfill can be tailored such that its thermal coefficient of expansion matches that of the solder so as to reduce relative movement and fatigue induced mechanical failure.
However, during curing, epoxies crosslink to form intractable networks that are difficult and impractical to rework when a faulty surface-mounted component is discovered. Epoxy-based adhesives are typically polymerized with various reactive agents such as anhydrides, aliphatic amines, aromatic amines, and mercaptans. The resulting polymers are tightly crosslinked networks that are solvent resistant and thermally stable. Consequently, the board to which the epoxy is applied is often discarded when a faulty surface-mounted device is discovered during burn-in or some other downstream process.
To make the adhesive reworkable, one method has been to develop a thermally degradable epoxy-based adhesive formulated by crosslinking an epoxide having a thermally-labile group within its structure with a curing agent. If rework is necessary, the cured adhesive is heated above a certain temperature such that the thermally-labile group integral to the epoxide cleaves thereby decomposing the adhesive network to facilitate removal of the defective component. Examples of thermally-labile groups introduced into the cured adhesive by the epoxide include alpha-alkoxy ester, oxycarbonyl, carbonate, and carbamate linkages.
Notwithstanding the state of the art, there is a need for epoxy-based adhesive compositions which provide the requisite physical properties when cured and at the same time are reworkable at a desired trigger temperature so as to allow for replacement of a device.